Candle-Power or Incandescent and Arc Lamps. 145 



The curves in fig. 7 represent graphically the result of the 

 observations, the ordinates reading candle-power and the 

 abscissae degrees of angle in a horizontal plane round 

 the lamp ; the zero angle, as before mentioned, corresponding 

 to that position of the lamp in which the axis of the loops 

 o^ the filament is parallel with the axis of the photometric 

 bench. 



The full-line curves show the variation in candle-power 

 in a horizontal plane, and the line of strokes gives the 

 M.H.C.P. 



These curves show in a very evident manner the very large 

 changes in candle-power which may occur in any lamp when 

 photometered in different positions. 



The sharp peaks and depressions which occur in some of 

 the lamps are particularly noticeable ; and with a view to 

 investigating the appearance of the filament when observed 

 in these directions, photographs of the lamp when incandescent 

 were taken in a few of these positions. 



These show that, as would be expected, the sudden 

 depressions in the curves are due to parts of the filament 

 becoming hidden, either behind another part of the filament, 

 or, in some of the multiloop lamps, behind the anchoring 

 support. 



The high peaks, on the other hand, are, as Mr. Russell * 

 pointed out some little time ago, due to the back of the bulb 

 acting as a mirror, the peaks of the curve corresponding to 

 the positions in which part of the filament happens to coincide 

 with the priucipal focus of the part of the bulb directly 

 behind it. The photographs, two of which are reproduced in 

 PI. III. fig. 8, show this very well indeed. Whilst all of 

 them show a large number of images of the filament in 

 the glass, the sharpness of these images proves that they 

 are at approximately the same distance from the camera 

 as the filament ; but in the cases where a peak occurs, and 

 only in such cases, the photograph shows a more or less 

 pear-shaped and blurred bright region ; indicating the 

 presence of an image which is not at the same distance. 



With regard to the relations between the mean spherical, 

 mean horizontal, and usually measured candle-power, the 

 results of the observations on the types of incandescent lamps 

 tested are shown in the following tables, in which the 

 following abbreviations have been used : — 



* " Mean Horizontal and Mean Spherical Candle-power,'' by Alex. 

 Russell, M.A.. Journ. Inst. Elect. Eng. vol. xxxii. p. J335. 



Phil Maq. S. »;. Vol. 0. No. 4!'. Jan. 1905. L 



